A rotational pulling method known as the CZ method is representative of methods for manufacturing silicon single crystals which are materials for silicon wafers. As is well known, in the manufacture of silicon single crystals by the CZ method, a seed crystal is immersed in a silicon melt formed in a quartz crucible, and while rotating both the crucible and the seed crystal, the seed crystal is pulled upward so as to grow a silicon single crystal below the seed crystal.
It is known that in silicon single crystals manufactured in this way, there occur various types of grown-in defects which cause problems for device formation processes. Two representative types of grown-in defects are dislocation clusters which occur in interstitial-silicon-dominant regions, and COPs or voids which occur in vacancy-dominant regions. Between these two regions is a ring-shaped OSF occurrence region. There are also vacancy type and interstitial-silicon type grown-in-defect-free regions. A representative defect distribution in a crystal radial direction is explained below using FIG. 1.
A ring-shaped OSF occurrence region exists in a ring shape at an intermediate position in a crystal radial direction. On an inside of the ring-shaped OSF occurrence region there is a COP or void occurrence region with a defect-free region intervening therebetween. On the other hand, on an outside of the ring-shaped OSF occurrence region there is a dislocation cluster occurrence region with an oxygen precipitation promotion region and an oxygen precipitation suppression region intervening therebetween. The oxygen precipitation promotion region is the vacancy type grown-in-defect-free region; the oxygen precipitation suppression region is the interstitial-silicon type grown-in-defect-free region.
Such a defect distribution is known to be controlled by the following two factors. The one is a pulling rate for the crystal; the other is a temperature distribution within the crystal immediately after solidification. Effects of the pulling rate for the crystal is explained as follows, using FIG. 2.
FIG. 2 shows a defect distribution in a vertical cross-section of a single crystal which is grown while gradually lowering the pulling rate. In a stage in which the pulling rate is higher, the ring-shaped OSF occurrence region is positioned in an outer circumference of the crystal. Therefore, in a wafer cut from a single crystal grown at a high pulling rate, COPs occur substantially in the entire radial direction of the crystal. As the pulling rate is lowered, the ring-shaped OSF occurrence region gradually moves toward a center portion of the crystal, and finally vanishes in the center portion of the crystal. Therefore, in a wafer cut from a single crystal grown at a low pulling rate, dislocation clusters occur substantially in the entire radial direction of the crystal. It should be noted that the crystal lateral cross-section of FIG. 1 corresponds to a cross-sectional view at position A in FIG. 2.
Both of dislocation clusters and COPs are harmful grown-in defects which worsen device characteristics, however COPs arc comparatively less harmful. Due to this and demands related to productivity, in the prior art, crystals are mainly grown at a high pulling rate for making the OSF occurrence region be situated on the outer circumference of the wafer or removing the OSF occurrence region outside the crystal, as indicated by a position at D or above in FIG. 2.
However, with marked trend toward miniaturization of integrated circuits in recent years, it begins to be noticed that COPs are harmful as well, and the need arises to prevent the occurrence of COPs as well as dislocation clusters. Technologies devised in response to this demand are to grow defect-free crystals by controlling point defect distributions, as described in Japanese Patent Application, First Publication No. 2001-220289 and Japanese Patent Application, First Publication No. 2002-187794.
Growth of crystals free of grown-in defects described in Japanese Patent Application, First Publication No. 2001-220289 and Japanese Patent Application, First Publication No. 2002-187794 utilize the phenomenon in which the defect distribution is controlled through the temperature distribution within the crystal immediately after solidification as described above.
That is, in a normal CZ method for pulling the crystal, beat is radiated from an outer surface immediately after solidification. Consequently, the axial-direction temperature gradient within the crystal immediately after solidification is such that a temperature gradient Ge in a peripheral portion tends to be greater than a temperature gradient Gc in a center portion. As a result, in a vertical cross-section of a single crystal grown by gradually lowering the pulling rate, the defect distribution, in particular the ring-shaped OSF occurrence region has a V-shape of convex downward with a pointed tip. As a result, even if the crystal is pulled at or in a vicinity of a critical pulling rate at which the ring-shaped OSF occurrence region vanishes in the center portion of the crystal, the grown-in-defect-free region is merely limited to the center portion of the crystal, and defects cannot be eliminated from all portions in the crystal radial direction.
It should be noted that while dislocation clusters and COPs do not occur in the defect-free region on the inside of the ring-shaped OSF occurrence region, dislocation clusters and COPs also do not occur in the ring-shaped OSF occurrence region itself, nor in the oxygen precipitation promotion region and the oxygen precipitation suppression region on the outer side. That is, these four regions are all grown-in-defect-free regions.
On the other hand, in the case in which the temperature of the crystal immediately after solidification, especially the outer surface thereof, is maintained in a positive manner by an improvement of a hot-zone structure in a crystal pulling furnace, the temperature gradient Gc in the center portion can be made equal to or greater than the temperature gradient Ge in the peripheral portion. Then in the vertical cross-section of a single crystal grown by gradually reducing the pulling rate, the shape of the OSF occurrence region still remains convex downward while the tip thereof being flattened, thereby the shape becomes a U-shape as shown in FIG. 3. In this state, by adopting pulling conditions to be at or in the vicinity of the critical pulling rate at which the OSF occurrence region vanishes in the center portion of the crystal, the entire portion in the crystal radial direction can be made defect-free. In FIG. 3, these conditions for pulling the crystal are within the B-C range.
Other technology for growth of crystals free of grown-in defects is hydrogen doping during pulling the crystal, as described in Japanese Unexamined Patent Application, First Publication No. 2000-281491 and Japanese Patent Application, First Publication No. 2001-335396. In these technologies, small amounts of hydrogen gas are intermixed with an inert gas fed into the pulling furnace, and the formation of vacancy defects can be suppressed as is the case with nitrogen doping of the silicon melt.
In the technologies to grow crystals free of grown-in defects by controlling the defect distribution, such as those disclosed in Japanese Patent Application, First Publication No. 2001-220289 and Japanese Patent Application, First Publication No. 2002-187794, it is necessary to choose, as the pulling conditions, low-pulling-rate conditions to be at or in the vicinity of the critical pulling rate at which the OSF occurrence region vanishes in the center portion of the crystal. Consequently, reduced productivity cannot be avoided.
In addition, a range of pulling rate (the margin; the range B-C in FIG. 3) for growing crystals free of grown-in defects is narrow, and it is difficult to grow a crystal free of grown-in defects stably. Consequently, it is difficult to obtain a crystal free of grown-in defects throughout the entire length of the crystal, thereby the production yield for the crystals free of grown-in defects is reduced. Hence there has been the problem that manufacturing costs of the crystals free of grown-in defects cannot easily be lowered. In particular, as the crystal diameter is increased to 200 mm and 300 mm, it becomes difficult to satisfy a condition of Ge≦Gc, and the range of pulling rate B-C for growing defect-free crystals tends to be even more narrow. Hence some breakthrough technique to circumvent such problems has been sought,
On the other hand, SOI (Silicon on Insulator) substrates enable semiconductor devices which operate faster and consume less power, and are hereafter expected to be in greater demand.
Major methods for manufacturing the SOI substrates include a bonding method in which a wafer with an oxide film is bonded to a normal wafer, and a SIMOX (Separation by Implanted Oxygen) method in which a buried oxide (BOX) layer is formed by implanting oxygen ions following by oxidation at a high temperature of 1300° C. or higher.
MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) formed on SOI layers in these SOI substrate have high radiation resistance and high latch-up resistance, and in addition to exhibiting high reliability, short-channel effects accompanying device miniaturization can be suppressed, and a low power consumption can be realized. Hence SOI substrates are regarded as high-functionality semiconductor substrates for use in next-generation MOS LSI devices. However, manufacturing processes are complex compared with those for ordinary wafers, and manufacturing costs tend to be higher, leading to strong demands for low-cost technologies.
The SIMOX substrate must be free of COPs inherently. That is, in the case in which oxygen ions are implanted in a substrate containing COPs, because COP areas are cavities, oxygen ions are scattered or arc implanted more deeply than in normal areas. Thereby, when the SIMOX substrate is annealed, irregularities Occur in the buried oxide (BOX) layer. For this reason, wafers are used for SIMOX substrates in which COPs are eliminated from a vicinity of a surface by an epitaxial growth or a high-temperature treatment in a hydrogen or argon atmosphere. But because the epitaxial growth, the high-temperature annealing, or some other additional processes are needed, there is a problem of high manufacturing costs.
With regard to the bonded SOI substrate, in the case in which COPs exist in a substrate positioned on an active-layer side, when manufacturing a thin film bonded SOI wafer having an active layer of 0.1 μm or less, areas containing COPs in the active layer becomes thinner. Thereby, the areas become pinholes which penetrate partially or completely through to the BOX layer, resulting in so-called HF defects and other faults. Consequently, it is necessary to use a wafer not containing COPs as the substrate on the active-layer side. Of course the same holds for a thick film bonded SOI having an active layer of greater than 0.1 μm, and in the case in which the active layer contains COPs, gate oxide integrity and device resistance separation become faulty. Therefore it is desirable that a wafer free of COPs be used as the active-layer side substrate.
From the above, because crystals free of grown-in defects are inherently free of COPs, when used as SOI substrates, there is no need for such additional processes as the epitaxial growth and the high-temperature annealing. Therefore, such crystals are promising for use as SOI substrates for which lower costs are demanded. However as explained above, the crystal free of grown-in defects is grown by pulling at a lower rate than that in the normal CZ method, and the range of pulling rate (margin) for growing crystals free of grown-in defects is exceedingly narrow. Therefore, production yields are low, and consequently it is difficult to reduce manufacturing costs of the crystal.
In order to solve these problems, it is necessary to raise the pulling rate so as to improve productivity, and further it is necessary to expand the range of pulling rate (margin) for growing crystals free of grown-in defects so as to stabilize crystal growth and improve production yields.
An object of the present invention is to provide a method for growing a silicon single crystal by which crystals free of grown-in defects are stably grown with good productivity in order to provide wafers that can be used as mirror-polished wafers or SOI wafers. Another object of the present invention is to provide a mirror-polished silicon wafer or a SOI wafer manufactured using such a method for growing a silicon single crystal and having high quality and low manufacturing costs, and methods for manufacturing the same.